Ambient light sensors may be used in conjunction with image displays in applications such as mobile phones and cameras, in order to adapt the display's backlighting level, thereby saving power. Current light sensors used in such applications typically have ±10% variation between parts. There is a need for ambient light sensors with improved consistency, while remaining cheap and simple to implement.
Known solutions to improved accuracy include:                Measuring the output during test and discarding those devices which are unsatisfactory. This gives a greatly reduced yield and thus increased manufacturing cost;        Measuring the output during test and adjusting a parameter, such as a resistor, by laser trimming. This adds to test cost. Also, the laser trim may re-form; and        Measuring the output during test and storing a correction factor in a non-volatile memory. This can only be used in circuits suitable for incorporating non-volatile memory, and adds to test cost.        
In addition, the latter two options perform calibration only at test, and do not allow for aging, temperature effects, etc.